In the kraft cooking process cellulosic material, most conveniently in form of chips, is treated at elevated temperatures with alkaline cooking liquor containing sodium hydroxide and sodium hydrogen sulfide. The fresh inorganic cooking liquor is referred to as white liquor, and the spent liquor containing the dissolved wood material is referred to as black liquor.
Since the initiation of kraft cooking processes to the present date, one of the most important objectives therein has been the attempt to reduce the energy consumption required to heat up the chips and chemicals. The method generally employed has been to recover heat energy at the end of the cooking process as it can then be used at the beginning of the process, as the chips and chemicals are brought together. In continuous cooking processes, this takes place by heating the chip material with secondary steam obtained from flashing the hot black liquor. In discontinuous, or batch cooking processes, however, the most useful technique is to use the recovered hot black liquor 1) as a direct heating media to be pumped into the digester and 2) to heat-up white liquor by means of heat exchangers.
In connection with this type of low-energy batch cooking, several methods for energy reclamation have been proposed. Some of these developments have resulted in industrial scale embodiments. Perhaps the most useful prior art method to date is that described in U.S. Pat. No. 4,578,149 by B. K. Fagerlund. This patent relates to an invention in which hot black liquor is displaced from the top of a batch digester to a particular hot black liquor accumulator by pumping wash filtrate into the bottom of the digester. This displacement into the accumulator is continued until the thermal displacement shows a clear drop in temperature after which the liquor is conducted to a separate tank for lower temperature black liquor. The reclamation of heat is then carried out by first pumping lower temperature black liquor into the next batch, and by then pumping hot black liquor from a hot black liquor accumulator, as well as hot white liquor warmed up by heat exchange with part of the hot black liquor into the batch. In this process the digester is brought up to a temperature approximately 20.degree. C. below the final cooking temperature, thus providing for a major portion of the energy required in the form of fresh steam for heating the liquor in conventional batch cooking processes. In general, this technology can be classified as a "Two Tank" concept, i.e.--one black liquor accumulator for "hot" liquor and another one for "lower temperature" liquor.
The development of batch cooking technology has thus been characterized by improvements in terms of energy savings therein. Very little attention has been paid, however, to other important issues in cooking technology, such as the effect and variability of the properties of recovered black liquors, uniform cooking conditions, uniform pulp quality, and the sensitivity of these operations to disturbances therein. As an example, such a critical operational necessity as the removal of soap that separates from black liquors has not even been mentioned in the prior low-energy batch literature. The failure to consider these issues, however, has to a great extent been responsible for the tedious and troublesome start-ups of some low-energy batch digesters as well as operation in less than optimal conditions, which results in disturbances, production losses and variability in the degree of cooking and in pulp quality.